In the fields of chemistry, biology, materials science, microelectronics, and optics, the development of devices that are small relative to the state of the art and conveniently and relatively inexpensively reproduced is important.
A well-known method of production of devices, especially in the area of microelectronics, is photolithography. According to this technique, a negative or positive resist (photoresist) is coated onto an exposed surface of an article. The resist then is irradiated in a predetermined pattern, and portions of the resist that are irradiated (positive resist) or nonirradiated (negative resist) are removed from the surface to produce a predetermined pattern of resist on the surface. This is followed by one or more procedures. According to one, the resist may serve as a mask in an etching process in which areas of the material not covered by the resist are chemically removed, followed by removal of resist to expose a predetermined pattern of a conducting, insulating, or semiconducting material. According to another, the patterned surface is exposed to a plating medium or to metal deposition (for example under vacuum) followed by removal of resist, resulting in a predetermined plated pattern on the surface of the material. In addition to photolithography, x-ray and electron-beam lithography have found analogous use.
In an article entitled “Materials for Optical Data Storage”, by Emmelius, et al., Angewandte Chemie, Int. Ed. (English), 28, 11, 1445-1600 (November, 1989), a review of methods of making CD/ROM, WORM, and EDRAW optical storage disks is presented. According to one method, photolithography is used to create a pattern of protrusions on a surface that can serve as a master for fabrication of articles that have a surface including a series of ridges and protrusions complementary to the photolithographically-produced master. These articles, including microridges and grooves at one surface, can be combined with other materials in a layered structure to form an optical storage device. An article in the Phillips Technical Review, volume 40, number 10 (1982), entitled “Manufacture of LaserVision Video Disks by a Photopolymerization Process”, by Haverkorn, et al., discusses similar technology. U.S. Pat. Nos. 5,170,461 (Yoon, et al.), 4,959,252 (Bonnebat, et al.) and 5,141,785 (Yoshinada, et al.) describe optical elements such as waveguides and optical recording media. Yoshinada, et al. describe a process involving coating a substrate with a polymer or prepolymer, pressing a contoured stamp into the polymer or prepolymer to create a contoured pattern in a surface of the polymer or prepolymer complementary to the contoured surface of the stamp, removing the stamp, and adding a reflective layer to the contoured surface of the polymer or prepolymer for use as an optical device.
Photolithographic techniques for fabricating surfaces with positional control of chemical functionalities at submicron resolution is described in an article entitled “Patterning of Self-Assembled Films Using Lithographic Exposure Tools”, by Dressick, et al., Jpn. J. Appl. Phys., 32, 5829-5839 (December, 1993). The technique involves exposure of a self-assembled film to deep UV irradiation through a mask. According to one technique, photochemical cleavage of an organic group occurs in exposed regions followed by chemical reactivity selectively at those regions.
Photolithography has found application in the biological arena as well. Sundberg, et al. describe a method for patterning receptors, antibodies, and other macromolecules at precise locations on solid substrates using photolithographic techniques in combination with avidin or streptavidin/biotin interaction in an article entitled “Spatially-Addressable Immobilization of Macromolecules on Solid Supports”, J. Am. Chem. Soc., 117, 12050-12057 (1995).
Reactive ion etching is a process that is useful in the semiconductor industry and other arenas for forming very small structures having a very high aspect ratio (a very high height/width ratio of features). Reactive ion etching is a dry process in which a gas is accelerated towards a surface to effect etching, in contrast to wet etching processes in which a liquid is simply allowed to contact certain regions of a surface and to chemically react at those regions. In wet etching processes, etching typically takes place not only in a direction perpendicular to the surface, but horizontally, as well. That is, with wet etching it can be difficult to etch relatively precise, vertical channels in a surface. Instead, the sidewalls of the channel are etched horizontally also. Reactive ion etching provides an advantageous alternative for etching channels with good, near-vertical sidewalls.
Reactive ion etching masks should have certain characteristics such as good hardness, inertness to the etchent species, and in many cases electrical insulating properties. Thus, materials suitable for reactive ion etching masks are limited. Many metal masks, such as gold masks, are unsuitable since the metals can sputter easily. Polymeric masks typically degrade under reactive ion etching conditions. A typical prior art reactive ion etching mask is made of silica and is formed by creating a layer of silica on a surface and etching the layer selectively to create a silica mask, using photolithography. Such procedures can be costly. In an article entitled “Poly(siloxane)-based Chemically Amplified Resist Convertable into Silica Glass”, by Ito, et al., Jpn. J. Appl. Phys., 32, 6052-6058 (1993), a poly(siloxane)-based chemically amplified resist is reported. A polymeric glass precursor is converted into silicate glass through a lithographic procedure.
Waveguides are generally defined by a core, surrounded by a cladding, that acts as a guide of electromagnetic radiation. The waveguide can propagate radiation via total internal reflection of the radiation within the core. Waveguides have served as important components of sensors and switches, and have been fabricated from a variety of materials including inorganic materials such as glasses and organic materials such as polymers. Polymeric waveguides have been fabricated using reactive ion etching, ultraviolet (UV) laser and electron-beam writing, induced dopant diffusion during polymerization (photo-locking and selective polymerization), selective poling of electro-optically active molecules induced by an electric field, and polymerization of self-assembled prepolymers. One common technique for forming polymeric waveguides is injection molding. For example, voids in a cladding material (or substrate) can be filled, via injection molding, with a core material. However, problems associated with this technique include softening and deformation of the cladding or substrate under temperatures required for injection molding. Fabrication with precision is compromised, typically. In an additional prior art technique, a polymeric film is spun onto a substrate and portions of the film are subsequently exposed to light by a photolithographic process, thereby changing the refractive index of a polymeric film and creating a waveguide in the film. This technique requires expensive and complicated photolithographic systems for base formation of the waveguide array, and subsequent multi-step processing is required such as removal of the polymeric film from the substrate, lamination processing, curing processing, and other processing steps.
U.S. Pat. No. 5,136,678 (Yoshimura) describe fabrication of an optical waveguide array by providing a clad substrate having a number of grooves arranged in lines on a surface of the substrate, the substrate being resistant to a UV-curable resin. A UV-curable resin is used to fill the grooves in the substrate and is UV cured to form a core material, and a covering clad portion is formed over the structure of a material that is the same as or close to the material used as the substrate “cladding”.
U.S. Pat. No. 5,313,545 (Kuo, et al.) describes a technique in which a two-part mold made of stainless steel, aluminum, ceramic, or the like is used to mold a polymeric waveguide core material via injection molding. The mold is opened via removal of the two portions, and the waveguide is placed in a second mold into which is injected a cladding material. Kuo, et al. report that a post-mold curing process is sometimes needed to maximize optical and physical qualities of core regions, support apparatus, and end portions.
U.S. Pat. No. 5,390,275 (Lebby, et al.) describe a method for manufacturing a molded waveguide. A first cladding layer is provided, and channels are formed in the first cladding layer. The channels in the first cladding layer are filled with an optically transparent polymer, and a second cladding layer is subsequently affixed over the channels thereby enclosing them.
U.S. Pat. No. 5,481,633 (Mayer) describes vertical coupling structures in which waveguide patterns include sections that lie in close proximity with other sections, for example one directly above another, such that the distance between coupling portions is very small and coupling between different guides can occur.
Biological and chemical interactions can be studied on the micro scale using combinatorial chemistry. This technique, as described in Chemical & Engineering News, 74, 7, 28-73 (Feb. 12, 1996), involves formation of different biological or chemical species in a grid pattern on a surface and used, for example, to screen compounds for potential biological or chemical activity. An article entitled “Combinatorial Chemistry-Applications of Light-Directed Chemical Synthesis”, by Jacobs, et al., Trends in Biotechnology, 12, 19-26 (January, 1994) describes a photolithographic process used in a spatially-addressable synthesis technique for forming a combinatorial library involving photolithography. A surface is derivatized with amine linkers that are blocked by a photochemically cleavable protecting group. The surface is selectively irradiated to liberate free amines that can be coupled to photochemically blocked building blocks. The process is repeated with different regions of the surface being exposed to light and involved in synthesis until a desired array of different compounds, in a grid pattern on the surface, is prepared. Each of these compounds then is assayed simultaneously for binding or activity. Binding “hits” can be identified by the particular location at which binding on the surface occurs.
While the above techniques represent, in some cases, useful advances in the art, many of these techniques require relatively sophisticated apparatus, are expensive, and generally consume more reactants and produce more by-products in collateral fabrication steps than is optimal, and/or lack optimal versatility in application. It is an object of the present invention to provide a variety of techniques for modifying a surface chemically and/or biologically at the micro and nanoscale, and to form very small scale structures, including waveguides and masks for etching processes conveniently, inexpensively, and reproducibly.